RNA-targeting small molecules represent a class of small molecules, organic compounds with traditional drug properties (e.g., Lipinski's rule of five) that can bind to RNA secondary or tertiary structures and alter translation patterns, localization, and degradation.
Recent discoveries implicating RNA in the pathogenesis of several forms of cancer and neuromuscular diseases have created a paradigm shift in drug discovery. This work combined with advances in structural characterization techniques such as NMR spectroscopy, X-ray crystallography, Cryogenic electron microscopy [1] together with computational modeling, [2] has pushed forward the realization that RNA is a dynamic yet viable drug target. Traditionally, RNA was thought to be a mediator between DNA sequence-encoded instructions and functional protein. However, recent reports have shown that there are a large number of non-coding RNAs (ncRNAs) that are not translated into protein. Whereas 85% of the human genome is transcribed into RNA only 3% of the transcripts code for functional protein. [3] Although, ncRNAs do affect gene expression [4] levels by a variety of mechanisms. [5] Further, RNA can adopt discrete secondary or tertiary structures which play a pivotal role in many biological processes and disease pathology. For these reasons, RNA is being recognized as an attractive drug target for small molecules.[ citation needed ]
The earliest attempts to target RNA led to the discovery that aminoglycosides could bind to human RNA. In an early report, Noller discovered that several classes of antibiotics (streptomycin, tetracycline, spectinomycin, edeine, hygromycin, and the neomycins) could "protect" nucleotides in 16S ribosomal RNA by binding to this RNA. [6] [7] Subsequent studies by Schroeder and Green began to plant the seed that RNA could be targeted. Schroeder uncovered that aminoglycosides could inhibit protein synthesis by interacting with the ribosome through interactions with the 3’ end of the 16S RNA of E. coli taking advantage of RNA conformational changes. [8] Green and coworkers further confirmed this idea, discovering that aminoglycosides blocked the interaction of HIV-1 Rev protein and its viral RNA-binding site. [9]
David Wilson and David Draper were the first to suggest that RNA structures could be targeted by small molecules. They hypothesized that RNA could be "druggable" by targeting the 3D structure in the same way as protein 3D structures are used as drug targets and furthered the idea that targeting RNA could be used to treat diseases. [10] Czarnik and co-workers at Parke-Davis completed a screen on HIV Tat. [11] They found multiple small molecule inhibitors of the HIV-1 Tat—TAR system that recognized the bulge, lower stem, or loop region of the TAR RNA. One of the compounds discovered, 2,4,5,6-tetraaminoquinozaline, binds to the loop region of TAR, downregulates cellular Tat transactivation, and ultimately inhibits HIV-1 replication. [12]
The use of aminoglycosides, while an early start to RNA-targeting, came with some challenges. These molecules were only modestly selective and showed unfavorable toxicity levels at relevant therapeutic concentrations. [3] As another strategy for targeting RNA, antisense oligonucleotides were developed which have been pushed forward through the clinic for several diseases. By this principle, if one can identify an RNA involved in disease then the sequence can be used to design a complementary antisense oligonucleotide, and that agent can be introduced into cells to treat the disease. [5] But, this approach in its basic form has been met with several challenges. The most obvious are their large size and propensity to degradation by nucleases. In order for cellular RNA to be effective it must enter the cells intact. While backbone modifications to antisense oligonucleotides in order to prevent nuclease degradation have been shown to work, this approach is still somewhat limited. Small molecules may present a better way to target RNA and subsequently DNA because they can be designed to be more "drug-like" and have a better chance of reaching their target, most by oral administration. For this reason, there is an emerging interest in designing and discovering small molecules to target RNA secondary and tertiary structures to ultimately treat new diseases.[ citation needed ]
There are limited examples of small molecules that target RNA and are approved drugs for the treatment of human disease. Ribavirin was approved in 2002 to treat Hepatitis C and viral hemorrhagic fever. As a nucleoside inhibitor, the guanosine analog prodrug is used to stop viral RNA synthesis and viral mRNA capping by incorporating into RNA and pairing to uracil or cytosine. Branaplam is currently in phase I/II clinical trial for the treatment of Spinal Muscular Atrophy (SMA). This molecule is from a class of pyridazine small molecules and enhances the inclusion of exon 7, resulting in a full-length and functional protein product.[ citation needed ]
Branaplam represents the first mechanistic study of splicing modulation using a sequence-selective small molecule. The drug stabilizes the transient double-stranded RNA (dsRNA) structure formed between the SMN2 pre-mRNA and U1 snRNP complex, a key component of the splicesome. Further, this compound acts by increasing the binding affinity of U1 snRNP to the 5’ splice site (5’ss) in a sequence-selective manner that is discrete from constitutive recognition. [13] Ataluren is in clinical trials for the treatment of Duchenne Muscular Dystrophy (DMD). It is believed that Ataluren acts by promoting insertion of near-cognate tRNAs at the site of the nonsense codon without affecting transcription, mRNA processing, mRNA stability, or protein stability to give nonsense suppression. This drug would be effective for ~10% of patients with DMD who have a single mutation in the DMD gene causing a stop codon to appear prematurely (nonsense mutation). [14]
Compared to the number of drug candidates that have successfully made it to the clinical trial phase, there are many more lead compounds which have been tested in vivo using various animal models. Much of the current work that has progressed to in vivo testing has been directed to the RNA repeat expansions implicated in genetic neuromuscular diseases. In myotonic dystrophy type 1 (DM1), r(CUG)exp mRNAs sequester proteins including the alternative splicing regulator MBNL1 into the nucleus causing missplicing. Several groups have developed compounds which bind the toxic RNA and dissolve nuclear foci. In 2011, Artero and coworkers discovered that a peptide could reduce the toxicity associated with r(CUG) repeats in Drosophila and mouse models. [15] Disney and colleagues provided the first small molecules that targeted r(CUG) repeats in animals models by using rational designing to identify many small molecules directly targeting this toxic RNA and the compounds improved disease defects in a DM1 mouse model. [16]
Other works by the Disney group has shown that in cellular models of various RNA-mediated diseases that are causes by RNA repeats such as r(CAG) in Huntington's disease [17] and r(CCUG) repeats in Myotonic Dystrophy Types 2 [18] [19] could also be targeted with small molecules. Nakamori and colleagues also reported in 2012 that erythromycin could be orally dosed in DM1 mouse models to restore missplicing defects and inhibit the complex formed between r(CUG) and MBNL1. [20] In that same year, Miller and coworkers screened a library of compounds to find a small molecule drug that could improve splicing defects in a mouse model. [21] The Zimmerman group has taken a rational design approach to discovering small molecule drugs that target r(CUG). One such compound contains a selective triaminotriazine recognition motif which binds to the UU mismatches in r(CUG) selectively most likely in a base triplet combined with an amidinium RNA groove binding unit. Studies using a Drosophila model for DM1 showed an influence on related phenotypic outcomes such as eye morphology and climbing distance. [22]
Aside from studies involved r(CUG) repeats, other complex RNA structures have also been targeted. Pearson and coworkers discovered that a cationic porphyrin (TMPyP4) bound a G-quadruplex r(G4C2) and inhibited the binding of proteins to r(G4C2). [23] Work by Disney and Petrucelli rationally identified small molecules that can target this repeat and affect disease biology in model cellular systems and also in patient-derive iNeurons. [24] Further studies by Rothstein and colleagues determined that TMPyP4 could suppress r(G4C2)-mediated neurodegeneration in a Drosophila model. [25] Additionally targets have been rationally identified by using a powerful seqecune-based design approach termed informal to identify dozens of bioactive small molecules that target disease causing non-coding RNA termed INFORNA. [26] This study important showed for the first time that small molecules appear to have selectivities that are competitive with oligonucleotides with cell-permeable and medicinally optimizable small molecules. Additionally, compounds have been shown to be bioactive in diverse disease settings that ranged from breast cancer. [26] [27] and hepatocellular carcinoma. [28] More recently, the Disney group further used their prediction database INFORNA to design Targaprimir-96 to target miRNA precursors in animal models of cancer, the first small molecules to do so. This compound has a nanomolar affinity for the miRNA hairpin precursor selectively over other sequences. Targaprimir-96 was further tested in cells and in mice, inhibiting tumor growth in a xenograft mouse model of triple negative breast cancer upon i.p. injection. [29]
RNA-targeting small molecule drug discovery has greatly benefitted from the available cellular models for disease. The use of cell culture in early development has become a requirement for assessing the basic efficacy of a drug candidate. Thus, more research groups have implemented these techniques in their programs. In a leading example, Al-Hashimi and coworkers identified six small molecules with high affinity for TAR of HIV-1 through a computational approach. They docked a library of small molecules onto RNA dynamic structures generated by NMR and Molecular Dynamics (MD) simulations. The hit molecules inhibited the Tat—TAR interaction in vitro. They arrived at lead molecule, netilmicin, that had the best selectivity for HIV-1 TAR and inhibited HIV-1 replication in cells with a low IC50. [30] The Disney group has studied aminoglycoside derivatives in 2009 for their ability to inhibit interactions between repeat RNA and proteins. Using their prediction database INFORNA, they discovered that a compound could bind to 1 x 1 UU internal loops on an N-methyl peptide backbone. They confirmed that like other compounds that target DM1 r(CUG), they could inhibit the complex between r(CUG)-MBNL1, disrupt nuclear foci, and increase nucleocytoplasmic transport of the gene in patient-derived DM1 fibroblasts. [31] In that study the Disney group also described several approaches to validate the RNA targets of small molecules. In the first approach termed chemical cross-linking and isolation by pull down (Chem-CLIP) and chemical cross-linking and isolation by pull down to map binding sites (Chem-CLIP-Map). [32] [33] [31]
These studies showed in cells that small molecules can direct target disease-causing r(CUG) repeats in DM1 and that impressively the compound can discriminate against other RNAs with shorter repeats and also between the mutant and wild type allele of the DMPK mRNA that contains r(CUG) disease-causing repeats. In an additional approach, dubbed small–molecule nucleic acid profiling by cleavage applied to RNA (Ribo-SNAP) [31] [34] showed that small molecules can be used to cleave RNA targets in cells and also importantly demonstrated that designer small molecules target precisely disease causing RNA repeats and discriminate against RNAs that are not disease causing but have short repeats of r(CUG). Thus, targeting RNA structure with small molecules can have important selectively discrimination implications in cells.[ citation needed ]
In 2014, Chenowith and colleagues reported a cationic triptycene scaffold that targets RNA and DNA three-way junctions. Subsequent studies showed that these molecules exhibited favorable cellular uptake and cytotoxicity in human ovarian cancer cell lines. [35] In 2017, the Xodo group reported anthrafurandione and anthrathiophenedione small molecules with aminoethyl side chains could bind to RNA G-quadraplexes at the 5’-UTR of certain mRNAs. Further, these compounds were shown to suppress the KRAS oncogene in pancreatic cancer cells and induce apoptosis by reducing the metabolic activity of the cells. [36]
In 2009, the Zimmerman group discovered a compound to target the trinucleotide repeat expanded RNA and DNA that cause DM1. Through rational design, they utilized a triaminotriazine recognition unit to target TT or UU mismatches through a Janus Wedge type binding mode, creating a base triplet with the mismatch. The combined use of an acridine intercalator to pi-pi stack on the target gave a nanomolar binding affinity for TT or UU mismatches over others. Along with high binding affinity, this molecule was shown to displace MBNL from the complex with r(CUG) with a micromolar Ki. [37] Additionally, HIV-1 RNA has been targeted extensively in vitro by RNA-binding small molecules. In 2007, Miller and coworkers used dynamic combinatorial chemistry to screen a compound library against HIV-1 frameshift regulatory stem-loop RNA. They identified a hit compound that was selective for the regulatory sequence with micromolar binding affinity. [38]
In 2011, Butcher and colleagues discovered a frameshifting stimulator (DB213) which bound to HIV-1 FS RNA with moderate binding affinity. An NMR structure of the RNA in complex with DB213, showed that the small molecule bound to the major groove of the RNA duplex. [39] Schneekloth and Hargrove have taken a different approach by targeting the HIV-1 TAR RNA hairpin. In a small molecule microarray screening, the Schneekloth group identified a thienopyridine derivative that interacts with HIV-1 TAR RNA hairpin. Further SAR studies provided more information on the structure and binding mode. The lead analogue was found to bind to the 5’-UTR of HIV with an IC50 of 40 μM for displacing a Tat-derived peptide. [40] The Hargrove group developed a small library of amiloride derivatives with changes at the C(5) and C(6) positions to improve the binding affinity of amiloride to the loop and bulge of the HIV-1 TAR RNA. Using in vitro studies and modeling, they found a hit compound whose inhibition activity was increased by more than 100x compared to the parent amiloride. This compound is reported to be one of the tightest non-aminoglycoside TAR ligands reported to date. [41]
In the fields of biochemistry and pharmacology an allosteric regulator is a substance that binds to a site on an enzyme or receptor distinct from the active site, resulting in a conformational change that alters the protein's activity, either enhancing or inhibiting its function. In contrast, substances that bind directly to an enzyme's active site or the binding site of the endogenous ligand of a receptor are called orthosteric regulators or modulators.
Oligonucleotides are short DNA or RNA molecules, oligomers, that have a wide range of applications in genetic testing, research, and forensics. Commonly made in the laboratory by solid-phase chemical synthesis, these small fragments of nucleic acids can be manufactured as single-stranded molecules with any user-specified sequence, and so are vital for artificial gene synthesis, polymerase chain reaction (PCR), DNA sequencing, molecular cloning and as molecular probes. In nature, oligonucleotides are usually found as small RNA molecules that function in the regulation of gene expression, or are degradation intermediates derived from the breakdown of larger nucleic acid molecules.
Drug design, often referred to as rational drug design or simply rational design, is the inventive process of finding new medications based on the knowledge of a biological target. The drug is most commonly an organic small molecule that activates or inhibits the function of a biomolecule such as a protein, which in turn results in a therapeutic benefit to the patient. In the most basic sense, drug design involves the design of molecules that are complementary in shape and charge to the biomolecular target with which they interact and therefore will bind to it. Drug design frequently but not necessarily relies on computer modeling techniques. This type of modeling is sometimes referred to as computer-aided drug design. Finally, drug design that relies on the knowledge of the three-dimensional structure of the biomolecular target is known as structure-based drug design. In addition to small molecules, biopharmaceuticals including peptides and especially therapeutic antibodies are an increasingly important class of drugs and computational methods for improving the affinity, selectivity, and stability of these protein-based therapeutics have also been developed.
Aptamers are oligomers of artificial ssDNA, RNA, XNA, or peptide that bind a specific target molecule, or family of target molecules. They exhibit a range of affinities, with variable levels of off-target binding and are sometimes classified as chemical antibodies. Aptamers and antibodies can be used in many of the same applications, but the nucleic acid-based structure of aptamers, which are mostly oligonucleotides, is very different from the amino acid-based structure of antibodies, which are proteins. This difference can make aptamers a better choice than antibodies for some purposes.
In genetics, trinucleotide repeat disorders, a subset of microsatellite expansion diseases, are a set of over 30 genetic disorders caused by trinucleotide repeat expansion, a kind of mutation in which repeats of three nucleotides increase in copy numbers until they cross a threshold above which they cause developmental, neurological or neuromuscular disorders. In addition to the expansions of these trinucleotide repeats, expansions of one tetranucleotide (CCTG), five pentanucleotide, three hexanucleotide, and one dodecanucleotide (CCCCGCCCCGCG) repeat cause 13 other diseases. Depending on its location, the unstable trinucleotide repeat may cause defects in a protein encoded by a gene; change the regulation of gene expression; produce a toxic RNA, or lead to production of a toxic protein. In general, the larger the expansion the faster the onset of disease, and the more severe the disease becomes.
In molecular biology, G-quadruplex secondary structures (G4) are formed in nucleic acids by sequences that are rich in guanine. They are helical in shape and contain guanine tetrads that can form from one, two or four strands. The unimolecular forms often occur naturally near the ends of the chromosomes, better known as the telomeric regions, and in transcriptional regulatory regions of multiple genes, both in microbes and across vertebrates including oncogenes in humans. Four guanine bases can associate through Hoogsteen hydrogen bonding to form a square planar structure called a guanine tetrad, and two or more guanine tetrads can stack on top of each other to form a G-quadruplex.
An enzyme inhibitor is a molecule that binds to an enzyme and blocks its activity. Enzymes are proteins that speed up chemical reactions necessary for life, in which substrate molecules are converted into products. An enzyme facilitates a specific chemical reaction by binding the substrate to its active site, a specialized area on the enzyme that accelerates the most difficult step of the reaction.
A trinucleotide repeat expansion, also known as a triplet repeat expansion, is the DNA mutation responsible for causing any type of disorder categorized as a trinucleotide repeat disorder. These are labelled in dynamical genetics as dynamic mutations. Triplet expansion is caused by slippage during DNA replication, also known as "copy choice" DNA replication. Due to the repetitive nature of the DNA sequence in these regions, 'loop out' structures may form during DNA replication while maintaining complementary base pairing between the parent strand and daughter strand being synthesized. If the loop out structure is formed from the sequence on the daughter strand this will result in an increase in the number of repeats. However, if the loop out structure is formed on the parent strand, a decrease in the number of repeats occurs. It appears that expansion of these repeats is more common than reduction. Generally, the larger the expansion the more likely they are to cause disease or increase the severity of disease. Other proposed mechanisms for expansion and reduction involve the interaction of RNA and DNA molecules.
Myotonic dystrophy (DM) is a type of muscular dystrophy, a group of genetic disorders that cause progressive muscle loss and weakness. In DM, muscles are often unable to relax after contraction. Other manifestations may include cataracts, intellectual disability and heart conduction problems. In men, there may be early balding and infertility. While myotonic dystrophy can occur at any age, onset is typically in the 20s and 30s.
An exonic splicing silencer (ESS) is a short region of an exon and is a cis-regulatory element. A set of 103 hexanucleotides known as FAS-hex3 has been shown to be abundant in ESS regions. ESSs inhibit or silence splicing of the pre-mRNA and contribute to constitutive and alternate splicing. To elicit the silencing effect, ESSs recruit proteins that will negatively affect the core splicing machinery.
Vicriviroc, previously named SCH 417690 and SCH-D, is a pyrimidine CCR5 entry inhibitor of HIV-1. It was developed by the pharmaceutical company Schering-Plough. Merck decided to not pursue regulatory approval for use in treatment-experienced patients because the drug did not meet primary efficacy endpoints in late stage trials. Clinical trials continue in patients previously untreated for HIV.
Myotonin-protein kinase (MT-PK) also known as myotonic dystrophy protein kinase (MDPK) or dystrophia myotonica protein kinase (DMPK) is an enzyme that in humans is encoded by the DMPK gene.
CUG triplet repeat, RNA binding protein 1, also known as CUGBP1, is a protein which in humans is encoded by the CUGBP1 gene.
Muscleblind Like Splicing Regulator 1 (MBNL1) is an RNA splicing protein that in humans is encoded by the MBNL1 gene. It has a well characterized role in Myotonic dystrophy where impaired splicing disrupts muscle development and function. In addition to regulating mRNA maturation of hundreds of genes MBNL1 autoregulate alternative splicing of the MBNL1 pre-mRNA transcript. The founding member of the human MBNL family of proteins was the Drosophila Muscleblind protein.
In molecular biology, Tat is a protein that is encoded for by the tat gene in HIV-1. Tat is a regulatory protein that drastically enhances the efficiency of viral transcription. Tat stands for "Trans-Activator of Transcription". The protein consists of between 86 and 101 amino acids depending on the subtype. Tat vastly increases the level of transcription of the HIV dsDNA. Before Tat is present, a small number of RNA transcripts will be made, which allow the Tat protein to be produced. Tat then binds to cellular factors and mediates their phosphorylation, resulting in increased transcription of all HIV genes, providing a positive feedback cycle. This in turn allows HIV to have an explosive response once a threshold amount of Tat is produced, a useful tool for defeating the body's response.
CCR5 receptor antagonists are a class of small molecules that antagonize the CCR5 receptor. The C-C motif chemokine receptor CCR5 is involved in the process by which HIV, the virus that causes AIDS, enters cells. Hence antagonists of this receptor are entry inhibitors and have potential therapeutic applications in the treatment of HIV infections.
Ming-Ming Zhou is an American scientist whose specification is structural and chemical biology, NMR spectroscopy, and drug design. He is the Dr. Harold and Golden Lamport Professor and Chairman of the Department of Pharmacological Sciences. He is also the co-director of the Drug Discovery Institute at the Icahn School of Medicine at Mount Sinai and Mount Sinai Health System in New York City, as well as Professor of Sciences. Zhou is an elected fellow of the American Association for the Advancement of Science.
Topological inhibitors are rigid three-dimensional molecules of inorganic, organic, and hybrid compounds that form multicentered supramolecular interactions in vacant cavities of protein macromolecules and their complexes.
RNA-dominant diseases are characterized by deleterious mutations that typically result in degenerative disorders affecting various neurological, cardiovascular, and muscular functions. Studies have found that they arise from repetitive non-coding RNA sequences, also known as toxic RNA, which inhibit RNA-binding proteins leading to pathogenic effects. The most studied RNA-dominant diseases include, but are not limited to, myotonic dystrophy and fragile X-associated tremor/ataxia syndrome (FXTAS).
In the management of HIV/AIDS, HIV capsid inhibitors are antiretroviral medicines that target the capsid shell of the virus. This is in contrast to most current antiretroviral drugs used to treat HIV, which do not directly target the viral capsid. These have also been termed "Capsid-targeting Antivirals", "Capsid Effectors", and "Capsid Assembly Modulators (CAMs)". Because of this, drugs that specifically inhibit the HIV capsid are being developed in order to reduce the replication of HIV, and treat infections that have become resistant to current antiretroviral therapies.